Neurotransmitters

Functional properties III



Neurotransmitters and their receptors

 Introduced in Medical Biochemistry

 Functional context – role in normal brain

 Role of particular pathways in disease

states



Reading: Review Chapter 5, pp. 88-93,

and read Chapter 6

Objectives



Distinguish between small molecule and peptide

neurotransmitters and understand the “life cycle” of

each



For each transmitter, know material presented in

lecture about synthesis, release, removal

mechanisms, receptor subtypes, and physiological

function



Understand basic relationship of neurotransmitter

dysfunction to addiction, schizophrenia and

depression

Criteria for establishing that a substance

is a neurotransmitter



Synthesized by the neuron and

stored in nerve terminals



Released by depolarization in a

calcium-dependent manner



Produces postsynaptic changes by

binding to specific receptors



Mimics action of endogenously

released transmitter when

administered near the synapse



Many neurotransmitters are termed

“putative” because all criteria are

not yet fulfilled

Two main groups of chemical transmitters



Small molecule transmitters (classical transmitters)

 Acetylcholine

 Amino acids: Glutamate, aspartate, GABA, glycine

 Monoamines (biogenic amines): dopamine,

norepinephrine, epinephrine, serotonin, histamine

 Purines (ATP and adenosine)



Peptide transmitters

 Short proteins consisting of 3-36 amino acid residues

 Over 100 peptides implicated as neurotransmitters

Life cycle of small molecule neurotransmitters (NT)

(steps illustrated on next slide)

1. Enzymes needed for NT synthesis are made in RER,

transported to Golgi, and modified (e.g., glycosylated)

2. Enzymes transported to nerve terminal by axonal

transport

3. Precursors taken up via transporter proteins on nerve

terminal and NT is synthesized in nerve terminal

4. Newly synthesized NT packaged into synaptic

vesicles by vesicular membrane transport proteins

5. Stimulation: release of NT by exocytosis

6. NT removed from synapse by plasma membrane

transporters (acetylcholine is an exception) and

diffusion

Life cycle of

small molecule

neurotransmitters

1

2

3

6

Diffusion, reuptake,

or degradation

5

4

Life cycle of peptide neuro-

transmitters (neuropeptides)

1

1. Polypeptides much larger than the final peptide

NT (pre-propeptides) made in RER; converted to

propeptide (signal sequence removed). Enzymes

needed for further processing also made in RER.

2. Propeptide and enzymes transported to Golgi,

packaged into vesicles.

2

3

3. These vesicles transported to nerve terminal by

axonal transport.

4. Enzymes cleave propeptide to produce smaller

peptide NT(s) that remains in vesicles (large

dense-core vesicles).

5. Stimulation: release of peptide NT by exocytosis.

4

6. Clearance: Peptide NT diffuses away and is

degraded by proteolytic enzymes.

6

5

Fast synaptic

effects versus

slow

modulatory

effects



Most transmitters have the ability to activate several receptor subtypes.

The nature of their synaptic effect depends on which receptor subtype

is present at the synapse.



Neuropeptides seem to produce only slow modulatory effects,

consistent with the identification of peptide receptors as G protein-

coupled receptors.



Some small molecule transmitters can activate both ionotropic and G

protein-coupled receptors (glutamate, GABA, serotonin, acetylcholine).

Thus, they can produce both fast effects and slow modulatory effects.

Others only activate G protein-coupled receptors and thus produce

only slow modulatory effects (dopamine, norepinephrine).

Transmitter co-localization



Used to be thought that each neuron used only one transmitter

substance



Now many examples of colocalization



Multiple neuropeptides (often occurs because distinct

peptides are synthesized from the same pre-propeptide – see

diagram above)



Neuropeptide with small molecule transmitter



Two small molecule transmitters

Co-transmitters need not be released simultaneously;

this increases the versatility of synaptic transmission



Peptide and small molecule transmitters are generally localized in

different types of vesicles (but there are exceptions; Table 6.1).



Small clear vesicles docked close to plasma membrane, large

dense core vesicles farther away.



Low frequency stimulation

raises Ca 2+ levels locally

near plasma membrane,

limiting release to small

molecule transmitters.



Prolonged high-frequency

stimulation increases the

Ca 2+ concentration

throughout the presynaptic

terminal, inducing slower

release of neuropeptides.

Electron micrographs showing different kinds of

synaptic vesicles



Small molecule

transmitter in small

clear vesicles (top)



Peptide transmitter in

large dense core

(bottom)

Co-transmitters: Functional implications?



The functional significance of transmitter co-

localization has been established in some systems.

For example, peptides co-released with glutamate

from primary sensory neurons act presynaptically to

increase their own release and the release of

glutamate, and can enhance the postsynaptic

response to glutamate.

Why does transmitter content matter so much?



Dictates function



Dictates chemical identity of the neuron



Not all neurons make all proteins. They are specialized to handle

the synthesis, release, metabolism and reuptake of a specific

transmitter (or a small number of colocalized transmitters). Thus,

they make the proteins necessary for these specific tasks.



For example, a dopamine neuron will make enzymes necessary for

dopamine synthesis (tyrosine hydroxylase) but not enzymes for

serotonin synthesis (tryptophan hydroxylase). It will make

transporters for the reuptake of dopamine, but not transporters for

the reuptake of GABA.



Selective expression of proteins is useful for identifying neurons

using histochemical techniques, e.g., dopamine neurons can be

identified using antibodies to tyrosine hydroxylase or to the

dopamine transporter.

Neurons make many types of

neurotransmitter receptors



This enables them to respond

to a variety of other kinds of

neurons.

Serotonin



For example, a dopamine

neuron might receive inputs

from serotonin, glutamate and

GABA nerve terminals.

Glutamate

GABA

DA



It will therefore synthesize

serotonin, GABA and

glutamate receptors.

Autoreceptors



Many neurons also synthesize

receptors that recognize their own

transmitter. These receptors are

called autoreceptors . For example,

dopamine receptors located on a

dopamine neuron would be termed

dopamine autoreceptors .

(-)



Autoreceptors on nerve terminals

enable feedback inhibition of

transmitter synthesis or release ( red

arrows ). Autoreceptors on the soma

or dendrites produce feedback

inhibitory effects on cell firing (when

activated, they slow firing rate).

(-)



Some autoreceptors, however,

produce positive feedback effects.

More in pharmacology…..

Neuropeptides



Over 100 peptides have been identified that probably

function as neurotransmitters



Will briefly consider 2 examples

 Opioid peptides

 Substance P



I will not say anything about their receptors except to

remind you that:

 Receptors for neuropeptides are G-protein

coupled receptors

 In keeping with this, peptides generally serve slow

modulatory functions

Opioid peptides



Important family of peptides, named because they bind

to same postsynaptic receptors activated by opium

(morphine)



Discovered in the 1970’s during a search for

endogenous compounds that mimicked actions of

morphine



Family of more than 20 opioid peptides that fall into

three classes: endorphins, enkephalins and dynorphins



In general, they depress synaptic transmission



Important for analgesia – but also for feeding,

aggression, sexual behavior, reward mechanisms,

addiction and more…

Substance P



Representative of the peptide family called the tachykinins



11 amino acids



Substance P plays different functions in different brain regions



Important example: Substance P-containing sensory neurons

in dorsal root ganglia - activated by noxious chemical, thermal,

mechanical stimuli. Function: Transmit information regarding

tissue damage to pain-processing areas in the CNS.



Capsaicin: Hot ingredient in hot chile peppers. Also active

ingredient in topical analgesic used for painful disorders such

as viral neuropathies (shingles) and arthritic conditions.



Capsaicin stimulates specific receptors on terminals of these

Substance P-containing neurons. Initially causes a burning

sensation, but with repeated activation, desensitizes the

receptors, reducing pain, and with prolonged use causes death

of these neurons.

Small-molecule neurotransmitters

Acetylcholine

systems



Peripheral nervous system:

Acetylcholine is transmitter

at the neuromuscular

junction (last lecture) and at

synapses in the ganglia of

the visceral motor system.



Central nervous system:

Two major groups of

cholinergic neurons (shown

in figure):

 The basal forebrain

constellation (red circle)

 Neurons in the

dorsolateral tegmentum

of the pons (black circle)

 Also: Cholinergic interneurons in caudate nucleus

Basal Forebrain Constellation



Located in telencephalon,

medial and ventral to the basal

ganglia.



Includes basal nucleus of

Meynert (shown) and medial

septal nuclei (not shown)



constellation provide cholinergic

innervation to the entire

neocortex, amygdala,

hippocampus, and thalamus.



These neurons, which are

important in memory and

cognition, degenerate in

Alzheimer’s disease. They will

be discussed further in the

lecture on that disorder.

Cells in basal forebrain

Cholinergic neurons in the dorsolateral

tegmentum of the pons



Project to the basal

ganglia, thalamus,

hypothalamus, medullary

reticular formation and

deep cerebellar nuclei

(many of these are not

shown)



Important in controlling

level of arousal - regulate

forebrain activity during

cycles of sleep and

wakefulness (more on

this in the sleep lecture)

Acetylcholine



All you need to know is….



Acetylcholine is synthesized

from choline and acetyl co-A

in a single reaction catalyzed

by choline acetyltransferase

(ChAT). This enzyme is a

useful marker for cholinergic

neurons.



Once released, acetylcholine

itself is not taken back up by

the presynaptic terminal. It

is degraded extracellularly

by acetylcholinesterase

(hydrolyzed back to choline).

The choline is taken up by

the presynaptic terminal and

recycled.

Acetylcholine can bind to two kinds of

receptors

1) Nicotinic acetylcholine

receptors



Ionotropic receptors



5 subunits around a central

membrane-spanning pore.

Acetylcholine binds, opens

channel to allow influx of Na,

efflux of K (Na effect dominates

because K is close to its

equilibrium potential). Produces

EPSP.



Named because they bind

nicotine (CNS stimulant in

cigarettes). When nicotine

repeatedly stimulates the receptor

(chronic smoker), the receptor

undergoes compensatory

adaptations that contribute to

addiction.



Nicotinic receptors mediate

transmission at the

neuromuscular junction, also

found in autonomic ganglia; less

prevalent in CNS

Toxins that interact with

nicotinic receptors



See Box 6A in Purves text (especially

graduate students – many of these

toxins are important research tools)



Example of a natural toxin: α -bungarotoxin, a peptide

component of snake venom. Blocks transmission at

neuromuscular junction, allowing snake to paralyze its

prey. Proved very valuable tool for purifying and studying

the nicotinic receptor.



Man-made toxins: Organophosphates – inhibit

acetylcholinesterase, allowing acetylcholine to accumulate

at the synapse. This depolarizes the postsynaptic cell

making it refractory to subsequent release, causing

paralysis. Some used as insecticides (insects are more

sensitive), others as chemical warfare agents (nerve gas).

Acetylcholine can bind to two kinds of

receptors, continued

Muscarinic acetylcholine receptors

 They are G protein-coupled receptors. There are at

least 5 subtypes (m1-5). They couple to a variety of

second messenger systems.

 These are most prevalent type of cholinergic receptor

in the CNS.

 Also found in ganglia of the peripheral nervous

system. Also mediate peripheral cholinergic

responses of autonomic effector organs – such as

heart, smooth muscle, and exocrine glands – and are

responsible for inhibition of heart rate by the vagus

nerve. (this bullet will not be tested in our course)

Excitatory amino acid transmitters



Glutamate is the most ubiquitous excitatory transmitter.

Nearly all excitatory neurons in CNS are glutamate

neurons, and over half of all brain synapses release

glutamate. Glutamate’s major role is in fast excitatory

transmission, but it also participates in plasticity and cell

death.

H

- OOC-CH 2 -CH 2 -C-COO -

NH 3 +



Aspartate also exerts fast excitatory actions, but

whether endogenous aspartate functions as a

transmitter in the brain remains controversial.

H

- OOC-CH 2 -C-COO -

NH 3 +

Glutamate nerve terminal



Most important precursor

for glutamate is glutamine,

supplied by glial cells



After release, glutamate is

taken up by transporters

(EATT) on presynaptic

terminals, postsynaptic

cells, and glial cells.



Multiplicity of uptake

pathways reflects

importance of maintaining

low extracellular glutamate

levels to avoid

excitotoxicity.

EATT=excitatory amino acid transporters

(usually this is EAAT)

Glutamate receptors

Ionotropic receptors (3 classes,

named for selective agonists)

 AMPA receptors

 NMDA receptors

 kainate receptors

Metabotropic receptors

(G protein-coupled receptors)



Ionotropic receptors

AMPA Kainate NMDA

Na + Na + Na + and Ca 2+

At least 8 subtypes

(mGluR1-8)



Divided into three major

families that work through

different second messengers

Metabotropic receptors



Produce slow postsynaptic

responses, can either

increase or decrease

excitability



Important in normal

transmission and neuronal

plasticity

AMPA receptors



These receptors mediate fast

EPSP’s at most glutamate

synapses in brain and spinal cord.



AMPA receptors are oligomers

formed by various combinations of

related subunits (GluR1-4).



Depending on the combination of subunits that make up

the final receptor complex, it will have somewhat different

pharmacological and kinetic properties.



Most AMPA receptors are permeable to Na + but not Ca 2 + .

(Important difference from NMDA receptors)



However, if GluR2 is not present, the AMPA receptor

channel will also pass Ca 2+ . (This bullet will not be tested,

but it increasingly important to understand the role of

GluR2-lacking AMPA receptors in plasticity).

NMDA receptors

Different than AMPARs in many ways:



Made up of different subunits

(different protein family).



Channel passes both Ca 2+ and Na +



Multiple modulatory sites



Glycine site : Glycine is

necessary co-agonist (both

glutamate and glycine must

bind).



Phencyclidine (PCP) binding site

(binds within channel; not

shown): responsible for

psychotomimetic (mimicking

psychosis) effects of PCP and

related drugs of abuse



Mg 2 + binding site : responsible

for Mg 2 + block

Most cells have both AMPA and NMDA receptors,

but they have different functions



AMPA receptors : Fast excitation.



NMDA receptors : Slower excitation

due to differences in the way the

channel works (but still faster than

GPCR’s)



So when NMDA receptors

participate in synaptic transmission,

they produce a late phase of the

EPSP.



However they often do not play

much of a role in normal

transmission. Instead, they are with

two special situations that we will

discuss later: plasticity and

excitotoxicity.

Why do NMDA receptors play only a limited role

in normal synaptic transmission?



Mg 2+ block: Mg 2+ binds to the

channel in a voltage-dependent

manner. At the normal resting

membrane potential (~65 mV), it

binds and blocks the channel, so

channel does not pass ions when

activated by glutamate.



However, if another stimulus (like

strong AMPA receptor activation)

depolarizes the cell (by 20-30 mV),

Mg 2+ is driven out of the channel.

Now, when glutamate binds, the

channel will pass Na + and Ca 2+ .

NMDA receptors are unique in being doubly gated



In cells with both NMDA and AMPA receptors, Mg 2+ block prevents the

NMDA receptor channel from contributing to EPSPs at the resting

membrane potential, so EPSP depends on AMPA receptors (LEFT).



However, if the neuron is depolarized by AMPA receptors, Mg 2+ is driven

out of the channel and the NMDA receptors begin to contribute to

synaptic transmission (RIGHT).

NMDA receptors are unique in being

doubly gated, continued



Thus, the NMDA receptor is unique in being a doubly gated channel : in

english, this means it will only open if two conditions are met: 1) transmitter

must bind, 2) and cell must be depolarized. This is a key feature for ideas

about its role in associative learning. This will be discussed further in Dr.

Eliot’s lecture on synaptic plasticity.

Why does the NMDA receptor have a

special role in plasticity and excitotoxicity?



First, what do these terms mean?



Plasticity: ability to change – here refers to ability of

synapses to change their strength (next lecture)



Excitotoxicity: death by over-excitation



NMDA receptor enables both because it has special

ability to pass Ca 2+ ions and thus activate Ca 2+ -

dependent signaling pathways that lead to both

phenomena



A little bit of NMDA receptor activation: plasticity



Too much: excitotoxicity

Excitotoxicity



In normal

synaptic

transmission,

glutamate levels

increase but this

is transient and

restricted to the

synaptic cleft.



In contrast, sustained and diffuse increases in glutamate

levels lead to neuronal death. This is called excitotoxicity

(death by over-excitation). Almost all neurons are

vulnerable because almost all neurons have glutamate

receptors.

Excitotoxicity, cont.



Excitotoxicity is an important cause of cell death

associated with ischemia (reduced blood flow to brain)

as a result of a stroke (occlusion of a cerebral blood

vessel). Evidence:

 Extracellular glutamate levels increase during

ischemia

 Glutamate receptor antagonists protect neurons from

ischemia-induced damage in experimental animals



Excitotoxicity also involved in other acute forms of

neuronal insult (hypoglycemia, trauma, epileptic

seizures) and perhaps chronic neurodegenerative

disorders (Alzheimers, ALS).

How does glutamate become involved in the

cascade leading to ischemic cell damage?



A key player is the Na/K pump, a major

consumer of ATP (20-40% of brain’s

energy consumption) – top panel



Performs the vital role of “re-setting” Na

and K gradients after action potentials



These gradients are critical for

generation of subsequent action

potentials, AND for maintaining the

activity of “opportunistic” transporters

that rely on these gradients to pump

other substances (e.g., glutamate)

against their own concentration gradients

(example on bottom: Na gradient used to

drive reuptake of a neurotransmitter).

Neuro-

transmitter

How does glutamate become involved in the cascade

leading to ischemic cell damage? (continued)



As a result of ischemia, ATP levels fall and

the Na/K pump can no longer perform its

vital role of pumping K in and Na out. This

allows the extracellular [K] to rise

excessively. High extracellular [K]

promotes increased glutamate levels in two

ways:

inside

outside

Na

K



First, it depolarizes neurons, so they fire

more action potentials . This increases

glutamate release. And, the occurrence of

action potentials produces further

disruption of ion gradients (which can’t be

reset because the Na/K pump isn’t

pumping). Ultimately, neurons become

incapable of firing action potentials.

Na

Glu

K

How does glutamate become involved in the cascade

leading to ischemic cell damage? (continued)



Second way that high extracellular [K]

promotes increased glutamate levels:

inside

outside



High [K] inhibits glutamate uptake - this is

because the Na and K gradients

maintained by the Na/K pump provide the

driving force for inward transport of

glutamate against its concentration

gradient.

Na

K

Na



When the gradients are not maintained, not

enough K inside, driving force for

glutamate uptake is lost. Extracellular

glutamate levels rise and further depolarize

surrounding neurons… results in vicious

cycle.

Glu

K

Once glutamate levels rise, how does it

kill neurons?



Two major mediators: Ca 2+ and free radicals (see next slide for

schematic diagram).



Ca 2+ damages cells by initiating biochemical cascades.

Activation of proteases and endonucleases leads to proteolysis

of microfilaments and destruction of DNA. Accumulation of Ca 2+

by mitochondria damages them and causes metabolic

uncoupling.



Ca 2+ also injures cells by increasing production of free radicals.

These are highly reactive molecules that interact with and

damage proteins, lipids and DNA. For example, Ca 2+ activates

enzymes (e.g., NOS, PLA 2 ) that promote formation of toxic

hydroxyl radicals. Cells have mechanisms to control free radical

damage; when these are overwhelmed, as occurs in

excitotoxicity, it is termed oxidative stress .

Intracellular

pathways

mediating

excitotoxicity

Glutamate antagonists as therapeutic

agents for ischemia?



A significant amount of ischemia-related neuronal death occurs only

after a delay: thus a window of opportunity for therapeutic

intervention. This has enormous potential significance for patients

brought into the ER for cardiac arrest, stroke or head injury.



In animal studies, both NMDA and AMPA receptor antagonists have

produced encouraging results, particularly in stroke models.



However, results of clinical trials with glutamate receptor antagonists

have been disappointing. Possible reasons:



Toxicity limits dosing



Animal models do not reproduce complexity and heterogeneity

of human strokes; drugs might work in a subset of patients?



Need for more prolonged administration in humans

(microdialysis studies suggest that vessel occlusion elevates

glutamate for 1 hr in rats but for >50 hrs in humans)



Excitotoxicity is only one mechanism by which ischemia kills

neurons – inflammation, apoptosis? (see Fig. 25.8 in Purves for

diagram of apoptosis)

Inhibitory amino acid transmitters



GABA is the major inhibitory transmitter in the brain and spinal cord,

but is absent in peripheral nerves. GABA is used as a transmitter

mainly by interneurons (projection neurons are usually glutamatergic),

although there are some important GABA projection neurons (e.g.,

striatal output neurons; see basal ganglia lectures).

+ H 3 N-CH 2 -CH 2 -CH 2 -COO -



Glycine is also an inhibitory transmitter

but its distribution is restricted compared

to GABA. It is implicated as an inhibitory

transmitter in the spinal cord, lower

brainstem and the retina.

H

H-COO -

NH 3 +

GABA nerve terminal



GABA is synthesized

from glutamate by

glutamic acid

decarboxylase (GAD).



After release, it is taken

up into presynaptic

terminals and glial cells

by GABA transporters.



GABA taken up into

presynaptic terminal can

be repackaged into

vesicles.



GABA taken up by glial

cells is “recycled” through

a more complex pathway

(which will not be tested).

Inhibitory transmitters: critical for normal

brain function and many disorders



Inhibitory transmission is critical for maintaining

normal activity in all neuronal circuits. For example,

normal cortical function depends on the activity of

diverse types of GABA interneurons that regulate the

activity of excitatory projection neurons.



Because of its importance in many circuits, GABA

neurons play a role in many disorders, e.g.,

Huntington’s chorea, Parkinson’s disease, senile

dementia, Alzheimer’s disease, and schizophrenia

GABA: Clinical considerations



A general rule is that drugs that diminish GABA

activity generally cause excitation leading to

convulsions, while drugs which enhance GABA

activity cause inhibition, leading to anticonvulsant

actions or sedation. Examples:



The synthetic enzyme for GABA (GAD) requires

vitamin B 6 as a coenzyme. Deficiency of vitamin B 6

leads to diminished GABA synthesis. This can

cause seizures and death in infants.



In epilepsy , a reduction in GABA-mediated inhibition

seems to be critical in establishment and spread of

focal seizures.



Seizures will be covered later in the course.

Two major classes of GABA receptors

GABA A receptor



Ionotropic receptor consisting of multiple subunits. Five classes

of subunits cloned, each with multiple isoforms. This is the basis

for the extraordinary functional diversity of GABA A receptors in

brain.



GABA A receptors are responsible for most fast IPSPs. The

receptor channel is selective for Cl - , so opening of channel

enables Cl - to enter the cell and produce a hyperpolarization.

GABA B receptor



G protein-coupled receptors, produce slow IPSPs



Some GABA B receptors are located on nerve terminals, where

they reduce the release of many transmitters through

presynaptic mechanisms.

GABA A receptors are important targets for drug action



GABA A receptors have many

modulatory sites where drugs act.



Benzodiazepines are anti-anxiety

agents and muscle relaxants (e.g.

valium). They increase the

frequency of channel opening in

response to GABA, so normal

GABA levels are more effective at

producing inhibition.



Barbiturates are used

therapeutically for anesthesia and

control of epilepsy (e.g.

phenobarbital). They prolong the

duration of channel opening.



Channel blockers like picrotoxin

are pro-convulsant. Picrotoxin

decreases channel open time.



Just know direction of the effect

not the mechanism (duration vs

frequency).

Glycine receptors



Glycine receptors are ionotropic receptors. They gate a

similar Cl - channel to that of the GABA A receptor and

produce similar fast inhibitory effects.



However, they lack the modulatory sites present on the

GABA A receptor.



Strychnine (toxin from plant seeds) acts by blocking

glycine receptors, causing overactivitiy in the spinal cord

and brainstem, leading to seizures. Used as rat poison.



Note: Glycine receptors are ionotropic receptors

activated by glycine. They are different than the

modulatory site on the NMDA receptor where glycine

binds as a co-agonist. This modulatory site is not

blocked by strychnine.

Monoamine transmitters (also called

biogenic amines)



This class includes dopamine, norepinephrine, serotonin, and

histamine (I will not cover epinephrine because it a very minor

transmitter in the CNS).



With one exception (5-HT 3 receptor), they exert modulatory effects

through G protein coupled receptors.



Each of the monoamines is produced by only a small number of

neurons, located in discrete clusters in the brainstem. However,

each cluster gives rise to extensive projections to the forebrain.

Thus, monoamines have an influence over behavior that is

disproportionately great considering the relatively small number of

neurons containing these transmitters.



As a result of their extensive projections, monoamines can

simultaneously modulate information processing in many different

brain regions. This enables them to influence “global” functions

such as attention, motivation, and mood. It also explains why

drugs which alter monoamine transmission (antidepressants and

antipsychotics) have such profound behavioral effects.

Catecholamine

synthesis



Dopamine, norepinephrine and

epinephrine are catecholamines

(contain catechol group - benzene

ring with two adjacent hydroxyl

groups- and NH3).



They are synthesized by a common

pathway. In all cases, the rate-

limiting step is hydroxylation of

tyrosine by tyrosine hydroxylase .



Please know that tyrosine

hydroxylase is the rate-limiting

enzyme, but other reactions will not

be tested; they are covered in

Medical Biochemistry.



Note: Serotonin and Histamine are

monoamines, but not

catecholamines.

Common features of catecholamines

(using DA nerve terminal as an example)



Synthesized; transported

into vesicle by VMAT

=

Dopamine

Transporter

Tyrosine



Vesicular release

L-Dopa



Reuptake by transporter

protein (presynaptic

terminal)

Dopamine

D2R



Postsynaptic receptors and

autoreceptors (all are

GPCRs except 5-HT3 R)

MAO

D1R D2R

COMT



Degraded by monoamine

oxidase (MAO) and

catechol-O-

methyltransferase (COMT)

COMT

=

Vesicular monoamine

transporter (VMAT)

Dopamine

pathways



Most DA cell bodies are located one of two midbrain cell groups:



Substantia nigra ( red ): These neurons project primarily to the

putamen and the caudate nucleus (nigrostriatal DA system).



Ventral tegmental area ( blue ): These neurons project to cortical

and limbic regions (mesocortical and mesolimbic DA systems).



Other DA systems:



Hypothalamus (tuberoinfundibular DA system)



Retina

Dopamine signals through G protein-

coupled receptors



Two major families (D1 and D2) defined based on

pharmacology and signal transduction.



Postsynaptic receptors are D1 & D2. Autoreceptors are

D2.



Other subtypes now cloned, but they exhibit either D1 or

D2-like pharmacology. The D2-like family includes D2,

D3 and D4, while the D1-like family includes D1 and D5.

The functional roles of subtypes within each family

remain unclear because of the lack of selective drugs

that distinguish D1 vs D5, or D2 vs D3 vs D4. However,

classical antipsychotic drugs are D2 receptor

antagonists.

Nigrostriatal, mesolimbic and

mesocortical dopamine pathways

Functional roles of dopamine projections



Nigrostriatal DA system (SN): Control of voluntary

movement. Its degeneration results in motor symptoms of

Parkinson’s disease. Also involved in motor side effects of

antipsychotic drugs (DA receptor blockers).



Mesolimbic DA system (VTA): Includes DA neurons

projecting to limbic regions (amygdala, septal area,

hippocampus) and the nucleus accumbens (ventral

striatum). Important in motivation, reward, drug abuse, and

aspects of schizophrenia.



Mesocortical DA system (VTA): Includes DA neurons

projecting to several neocortical regions and most densely

to prefrontal cortex. Prefrontal cortex is involved in

motivation and planning, the temporal organization of

behavior, attention, and social behavior. This system may

be dysfunctional in schizophrenia.

Functional roles of dopamine projections

Continued



Tuberoinfundibular DA

system: Originates in

arcuate nucleus and

projects to infundibular

stalk, where DA released

into capillaries of the

pituitary portal system

inhibits prolactin secretion

by the anterior pituitary.

Drugs of abuse

and monoamine

neurons



DA is an important transmitter for motivation and reward

related to natural reinforcers (food, water, sex). The “high”

produced by cocaine and amphetamine reflects their ability

to hijack normal reward mechanisms by raising DA levels in

reward-related brain regions such as the nucleus

accumbens and prefrontal cortex.

Addiction results from complex adaptations in

neuronal pathways that mediate motivated behaviors



Both cocaine and amphetamine interact

with all monoamine transporters,

although effects on DA are believed to be

most important for addiction. Cocaine

acts by blocking reuptake (red X ).

Amphetamine also blocks reuptake and

also increases release by making

transporters run backwards. Both drugs

increase the time that DA and other

monoamines can interact with receptors

in the synapse.

Tyrosine

L-Dopa

Dopamine

Transporter

Dopamine

D2R

X



Repeated exposure to amphetamine or

cocaine results in a cascade of long-

lasting changes in the function of DA

neurons and other neurons in neuronal

circuits related to motivation and reward.

These changes are responsible for

addiction.

D1R D2R

Addiction

Norepinephrine pathways:

Two major clusters of NE cell bodies:



Locus ceruleus : located beneath floor of 4 th ventricle in the rostrolateral

pons. Origin of ascending NE projections.



Lateral tegmental system : cell groups in the rostral medulla, including

dorsal motor nucleus of the vagus, nucleus of the solitary tract, and the

medullary reticular formation. Origin of descending NE projections.

Functional roles of norepinephrine projections

NE neurons of the locus ceruleus

have very diffuse projections.

Innervate nearly all parts of

cerebral cortex, plus basal

forebrain, hippocampus, amygdala,

thalamus and cerebellum.

 Functional roles: Regulating

level of arousal (more in sleep

lecture); attention and

vigilance; responding to stress;

and depression

NE neurons of the ventral lateral

tegmentum (medulla) contribute to

integration of autonomic function in

brain stem and spinal cord.

NE receptors: Two major classes are defined by pharmacology (α and β),

but each has multiple subtypes. Presynaptic receptors (autoreceptors)

modulate NE release.

Serotonin



Serotonin (5-HT) is an

indoleamine, because it

contains an indole group.



It is synthesized by a

different pathway than the

catecholamines, but there

are similarities.



The precursor is an amino

acid (tryptophan) and the

rate-limiting step is

hydroxylation of the amino

acid (by tryptophan

hydroxylase).



Not necessary to memorize

synthetic reactions.

Serotonin (5-HT)

pathways



Serotonin neurons are

found at most levels of

the brainstem (pons,

midbrain, medulla),

concentrated in the raphe

nuclei (raphe means

“seam” in greek).



Projections from the rostral raphe nuclei travel to the

forebrain and provide diffuse and extensive serotonergic

innervation of cerebellar cortex, striatum, limbic structures,

olfactory tubercle, hippocampus, and the diencephalon.



The caudal raphe nuclei provide most of the serotonergic

innervation of brainstem and spinal cord.

Functional roles of serotoninergic

projections



Like NE neurons, 5-HT

neurons give rise to

widely branched axons

that innervate most of the

CNS. But NE and 5-HT

emphasize different

cortical areas and layers.



Activity of both NE and 5-

HT neurons fluctuates

with sleep/wakefulness,

so both play a role in

regulating level of arousal.